Improved <i>k</i>‐space trajectory measurement with signal shifting

Beaumont, Marine; Lamalle, Laurent; Segebarth, Christoph; Barbier, Emmanuel

doi:10.1002/mrm.21254

Cited by 27 publications

(25 citation statements)

References 8 publications

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“…[10], and collecting additional data to enable the cross-term coefficients to be determined. Ultimately, this would increase both the data acquisition time and the coefficient computation time, as far more data would need to be sampled and processed.…”

Section: Discussionmentioning

confidence: 99%

“…,G͒ [10] In practice, T A is known and is assumed to be independent of the gradient amplitude. If data are acquired for N S slices, with the s th slice centered at x s , and N G gradient amplitudes for each slice location, with the g th gradient amplitude of G g , then the associated system delay and linear and B 0 eddy currents coefficients can be found by solving the following constrained minimization problem:…”

Section: Theorymentioning

confidence: 99%

“…[10] and [12] to describe the phase in the MR signal was explored by comparing ͑x s ,t,G g ͒ Ϫ ˆ͑x s ,t,G g ͒ and ͑x s ,t,G g ͒ Ϫ ˜͑x s ,t,G g ͒ and noting that the perfect model would result in a zero-mean residual. The system constants and apparent system delay were measured multiple times (n ϭ 5) in separate sessions spread over 15 days, during which a cryogen fill was performed.…”

Section: Determination Of System Constants and Apparent System Delaymentioning

confidence: 99%

“…A potential solution is to measure the actual k-space trajectories (7)(8)(9)(10)(11)(12) and eddy currents (12) and to use them to accurately reconstruct the acquired data. While such measurement approaches work well in principle, all unique gradient waveforms must be mapped, which is time-consuming for acquisitions with many different waveforms such as TPI (13) and 3D Cones (14).…”

mentioning

confidence: 99%

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Characterization and correction of system delays and eddy currents for MR imaging with ultrashort echo‐time and time‐varying gradients

Atkinson

Thulborn

2009

Magnetic Resonance in Med

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Reconstruction of high-quality MR images requires precise knowledge of the dynamic gradient magnetic fields used to perform spatial encoding. System delays and eddy currents can perturb the gradient fields in both time and space and significantly degrade the image quality for acquisitions with an ultrashort echo time or with rapidly varying readout gradient waveforms. A technique for simultaneously characterizing and correcting the system delay and linear-and zero-order eddy currents of an MR system is proposed. A single set of calibration scans were used to compute a set of system constants that describe the effects of system delays and eddy currents to enable accurate reconstruction of data collected before uncorrected eddy currents have decayed. Accurate MR image formation requires precise control of the gradient magnetic fields to traverse a defined k-space trajectory during data acquisition. System delays and eddy currents can cause the actual trajectory to deviate substantially from the ideal trajectory (1,2). Modern MRI systems include preemphasis hardware for eddy current compensation (3-5) that correct linear (first-order) eddy currents by altering the digital gradient waveform (1) or high-pass filtering the gradient waveform (6) and correct B 0 (zero-order) eddy currents by modulating the receiver phase (5) or using a current-controlled B 0 coil. Because preemphasis systems are limited to a small number of time-constants, it is not uncommon for short-time constant eddy currents to remain even when they are optimally configured. System delays (e.g., amplifier group delay) are typically determined during manufacturing or installation using calibration scans and manual tuning. However, because eddy currents with time-constants shorter than, or comparable to, the echo-time cannot be distinguished from a true delay (1), the measured delay value is more properly termed the apparent system delay.Traditional Cartesian acquisitions are inherently robust to uncorrected delays and short time-constant eddy currents, particularly when the echo-time is long and the data are primarily acquired under a constant gradient amplitude after all uncorrected eddy currents have decayed. These conditions are often met when preemphasis is used to compensate long time-constant eddy currents, enabling high-quality images to be obtained with minor retrospective data corrections. In contrast, acquisitions that use an ultrashort TE (UTE) or traverse complex k-space trajectories using high slew-rate, time-varying readout gradient waveforms (e.g., Twisted Projection Imaging [TPI], 3D Cones) sample data while uncorrected eddy currents are present, distorting the actual k-space path, and may have an undefined center of k-space due to the unknown system delay, leading to significant image degradation (2).A potential solution is to measure the actual k-space trajectories (7-12) and eddy currents (12) and to use them to accurately reconstruct the acquired data. While such measurement approaches work well in principle, all unique gradient w...

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Section: Discussionmentioning

confidence: 99%

Section: Theorymentioning

confidence: 99%

Section: Determination Of System Constants and Apparent System Delaymentioning

confidence: 99%

mentioning

confidence: 99%

See 2 more Smart Citations

Characterization and correction of system delays and eddy currents for MR imaging with ultrashort echo‐time and time‐varying gradients

Atkinson

Thulborn

2009

Magnetic Resonance in Med

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“…Beaumont et al (9) recognized that for the methods of Zhang et al (7) and Duyn et al (8), low or zero amplitude values would occur at k-space locations farther from the origin than a distance equal to the inverse of the slice thickness. The slice thickness must therefore be kept very small to obtain accurate estimates of larger k-space values.…”

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confidence: 99%

Fast, simple gradient delay estimation for spiral MRI

Robison

Devaraj

Pipe

2010

Magnetic Resonance in Med

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Timing delays between data acquisition and gradient transmission result in image degradation. This is especially true in spiral MRI, where delays can alter data in a nonuniform manner, generating significant artifact in the reconstructed data. The many methods that exist to mitigate these delays or measure the k-space coordinates require long measurement times, complicated analysis, specialized phantoms or hardware, or significant changes to the sequence of interest. A fast and simple method is proposed to measure delays on each gradient channel. It requires only minimal modification to an existing spiral sequence and can be used to measure independent delays on three gradient channels and any scan subject within six sequence repetition times. Accurate reconstruction of MRI data requires knowledge of the k-space sample locations. Deviations from the theoretical gradient waveforms can be caused by eddy currents, gradient amplifier nonlinearities, and other system imperfections. These deviations take the form of gradient timing delays and amplitude distortions that result in altered sample locations in k-space, producing artifacts in the reconstructed images. In spiral trajectories, these artifacts can be mistaken for aliasing, loss of resolution, poor signalto noise-ratio, or off-resonance blurring, thus making the source of the artifacts difficult to discern.Modern MRI scanners mitigate eddy currents through the use of gradient coil shielding and gradient waveform preemphasis. Sequences that place large gradient slew-rate and amplitude demands on the hardware can induce eddy currents, despite gradient coil shielding. Gradient waveform pre-emphasis provides an additional level of eddy current compensation but can only be optimized for a limited range of sequences. A number of techniques have been employed to compensate for the remaining deviations from the expected k-space coordinates.Several strategies seek to measure the actual gradient waveform for correct reconstruction or improved gradient pre-emphasis. Spielman and Pauly (1) directly measured the current on each gradient channel. Errors from the gradient amplifier were directly measured, but eddy currents were assumed to be negligible. Onodera et al. (2) proposed

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Spiral imaging artifact reduction: A comparison of two k‐trajectory measurement methods

Lechner

Sipilä

Kerr

et al. 2009

Magnetic Resonance Imaging

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Purpose:To compare an external sensor-based k-space calibration technique with a routine precalibration method for quantification of method accuracy and reduction of spiral imaging artifacts to obtain improved image quality. Materials and Methods:Recently, magnetic field monitoring (MFM) has been introduced as a new calibration technique of gradient field-related imperfections. External sensors are placed near the observed object to measure magnetic field variations during image acquisition. The measured field data are used to determine the actual kspace trajectory and for image reconstruction to reduce artifacts. In the past, precalibration techniques have been proposed where the k-space trajectory is measured by means of special calibration sequences directly in the object of interest. In this study, MFM is introduced as an effective correction technique for spiral imaging. On the basis of a comparison, whether MFM is as viable as the chosen reference method presented by Duyn et al (J Magn Reson [1998] 132:150 -153) is analyzed in terms of detecting imperfections of spatial encoding gradients in order to correct for these in image reconstruction. As this technique is used as a reference method, it is given the acronym Duyn calibration technique (DCT). MFM and DCT are compared and timing delays, k-space offsets, eddy current effects, k-trajectory error propagation, image distortions, and signal-to-noise ratio were determined for different spiral sequences in two different phantoms.Results: Both techniques effectively detect k-space offsets and k-trajectory error propagation and correct for general error sources like timing delays. In object border areas, artifacts such as deformations and blurring were dramatically reduced. Within all tested categories, MFM performed as well as DCT. In terms of k-trajectory error propagation and image distortion quantification, MFM was more accurate. Conclusion:We introduce MFM as an effective and accurate correction technique for spiral imaging, where a comparison of MFM and DCT has shown that both techniques are accurate correction techniques for spiral imaging.

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Improved k‐space trajectory measurement with signal shifting

Cited by 27 publications

References 8 publications

Characterization and correction of system delays and eddy currents for MR imaging with ultrashort echo‐time and time‐varying gradients

Characterization and correction of system delays and eddy currents for MR imaging with ultrashort echo‐time and time‐varying gradients

Fast, simple gradient delay estimation for spiral MRI

Spiral imaging artifact reduction: A comparison of two k‐trajectory measurement methods

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